1. Field of the Invention
This invention relates to a multi-layer information medium which has at least two information-storing layers such as recording layer, and a method for making such medium.
2. Prior Art
There is a growing need for an optical disk having a higher density and a higher capacity. DVD (Digital Versatile Disk) is already commercially available, and the DVD has a storage capacity of about 4.7 GB per single side which is about seven times larger than the compact disk. Technologies enabling further increase in the amount of information recorded have been actively developed.
Technologies that have been used for increasing the recording capacity of an optical disk include use of a recording/reading beam having a shorter wavelength, use of an objective lens having a higher NA (numerical aperture) in the optical system irradiating the recording/reading beam, increase in the number of recording layers, and multi-value recording. Among these, three-dimensional recording by increasing the number of recording layers enables remarkable increase in the recording capacity at low cost compared to the use of a shorter wavelength or use of a lens with a higher NA. The three dimensional recording medium is described, for example, in Japanese Patent Application Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses a medium wherein a rewritable information storage layer and a read only information storage layer are laminated.
In a multi-layer recording medium, a transparent resin layer which is transparent to the recording/reading beam is generally provided between adjacent recording layers, and the recording/reading beam reaches the recording layer after passing through the transparent resin layer and returns to the optical pick up after being reflected at the surface of the recording layer. Accordingly, the transparent resin layer is required to exhibit strictly uniform thickness, quality, and optical properties. When the medium has a disk shape and the transparent layer comprises a resin, the transparent layer is generally formed by spin coating since the spin coating is a process which is capable of forming a relatively uniform transparent layer. The transparent layer formed by the spin coating, however, suffers from the problem that the layer is thicker in the outer peripheral region of the disk compared to the inner peripheral region. In other words, the layer suffers from the problem of inconsistent thickness in the radial direction of the disk. The number of the transparent layers increase with the number of the recording layers, and such thickness inconsistency is accumulated with the increase in the number of the data layers. As a consequence, even if the recording/reading beam entered the medium in the outer peripheral region of the disk at normal direction, the data beam reflected at the surface of the recording layer will not be normal to the medium surface, and the quantity of the light returning to the optical pickup will be reduced. The medium will then exhibit different outputs in the inner peripheral region and in the outer peripheral region.
In the case of a joined optical recording disk wherein the recording layer has been sandwiched between two substrates, increase in the thickness of the transparent resin layer in its peripheral region is likely to result in the warping or distortion of the optical disk. Improvement in the mechanical precision would be then difficult.
In the reading of a multi-layer recording medium including a plurality of recording layers by using an optical pickup which emits a reading beam, the optical pickup receives the beam reflected from the recording layer on which the reading beam had focused, and in addition, the beam reflected from the recording layer(s) other than the recording layer to which the reading beam had focused. This results in the signal interference between the plurality of recording layers, and cross talk is induced. As a consequence, noise is introduced in the signal that has been read. The influence of the beam reflected from the recording layer other than the target recording layer reduces inversely with the square of the distance between the recording layers. Therefore, increase in the distance between the adjacent recording layers is effective in reducing the noise induced. For example, when the medium is used with an optical pickup having the structure normally employed in DVD and other conventional optical disks, the recording layers are disposed at a mutual distance of at least 30 xcexcm, and preferably at least 70 xcexcm to realize the signal quality of practically acceptable level. This corresponds to the Examples of the JP-A 198709/1997 wherein a transparent resin layer of 100 xcexcm thick is provided between the recording layers, and the JP-A 255374/1996 wherein two adjacent information storage layers are disposed at a distance of 30 xcexcm or more.
However, when the distance between the adjacent recording layers is increased to as large as 30 xcexcm or more, limitation in the number of recording layers in the medium will be required to avoid excessive increase in the disk thickness, and the total storage capacity of the disk will also be limited. In addition, formation of a transparent resin layer with a consistent thickness of 30 xcexcm or more is difficult. The thick resin layer also suffers from increased internal stress and the medium will be subject to warping. As a consequence, reliable provision with the optical disk of the required mechanical precision has been difficult.
In view of such situation, JP-A 222856/1998 and SOM ""94 technical digest (1994) 19, for example, propose use of an optical pickup provided with a confocal optical system which utilizes the principle of a confocal microscope for the reading of each recording layer in order to reduce the cross talk between the recording layers of the multi-layer recording medium. In the case of an optical pickup provided with a confocal optical system, a pinhole is provided in the optical system and the reading is accomplished by using the beam that had passed thorough this pinhole. Accordingly, when an optical pickup provided with a confocal optical system is used, the range of the focus servo is reduced, and a higher thickness consistency is required for the transparent resin layer.
A multi-layer recording medium also suffers from the problem as described below. In the case of the medium having a single recording layer formed on a substrate, the shape of the grooves (guide grooves) formed in the resin substrate will be transferred to the recording layer. In contrast, in the case of a medium wherein two or more recording layers are formed on the substrate with an intervening relatively thick transparent layer between the recording layers, it is quite difficult to transfer the shape of the grooves formed in the substrate to all of the recording layers since the groove depth is about 100 nm at most for optical reasons while the distance between the recording layers is at least 30 xcexcm as described above. As a consequence, formation of the grooves in the transparent resin layer by photopolymerization (2P) process will be required as described, for example, in the JP-A 198709/1997 and an eminent increase in the production cost is invited.
An object of the present invention is to realize excellent reading properties in all of the information-storing layers in a multi-layer information medium having a plurality of information-storing layers formed therewith. Another object of the invention is to realize good mechanical precision in the multi-layer information medium. A further object of the invention is to provide such multi-layer information medium at a low cost.
Such objects are attained by the present invention as described in (1) to (5), below.
(1) An optical information medium having at least two annular information-storing layers each storing recorded information and/or servo information on a disk-shaped substrate formed with a center hole or between a pair of disk-shaped substrates each formed with a center hole, wherein at least one of the information-storing layers is recorded or read by the recording beam or the reading beam which has passed through other information-storing layer(s), wherein
the medium has at least two annular resin layers each having an annular raised rim at its inner periphery, and the annular resin layers are formed such that their inner peripheries are in the form of steps and the annular raised rim of the resin layer is prevented from being covered by the overlying resin layer.
(2) An optical information medium according to the above (1) wherein the resin layer is disposed between adjacent information-storing layers each storing the recorded information, and the resin layer has a thickness of 5 xcexcm to less than 30 xcexcm.
(3) An optical information medium according to the above (1) or (2) wherein said information-storing layer is read by using a confocal optical system.
(4) An optical information medium according to any one of the above (1) to (3) wherein the information-storing layer comprises a data layer storing the recorded information and a servo layer storing servo information which are independently formed from each other.
(5) A method for producing the optical information medium according to any one of the above (1) to (4) wherein the method comprises the consecutive steps of:
placing the substrate on a turntable and covering the center hole of the substrate by a plug means having a disk member, supplying a coating solution containing the resin onto the disk member, and rotating the substrate with its center hole covered by the plug means to thereby spread the coating solution over the substrate and form the resin layer;
removing the disk member from the substrate to leave the annular resin layer having an annular raised rim formed at its inner periphery; and
curing the resin layer.